Multiphase pipelines are used in the oil and gas production industry for transporting multiphase fluid streams comprising a liquid phase (typically, comprising oil and/or gas condensate and produced water) and a gaseous phase from offshore production wells to a nearby surface production facility.
Typically, a pipeline located on the seabed may be connected to an ascending underwater flow line, known as a “riser”, which conveys the produced fluids up to the surface production facility, preferably, to a gas-liquid separator for separating the gaseous phase from the liquid phase.
In certain pipeline/riser configuration “slugging” flow can occur. This condition refers to the situation where the flow rate of the separate gaseous and liquid phases from out of the top of the riser is not steady and oscillates between largely (or all) gas and largely (or all) liquid. This situation can lead to three significant problems, typically:
1) Flooding of the inlet gas/liquid separator vessel due a large “slug” of liquid arriving, the volume of which is more than the available liquid capacity in this vessel. This may lead to emergency shutdowns.
2) Unsteady operating conditions in the topside equipment. This can lead to failure to meet product specifications; typically failure to meet the regulations for the amount of oil contaminating the produced water discharge, and failure to meet the specified water content in the export oil, both owing to difficulties in achieving a stable oil/water interface. In addition, this can lead to problems in controlling the gas compressors of the surface production facility owing to the unsteady flow of feed gas to such units from the gas/liquid separators, which may result in some flaring of gas.3) High stress loads may be imposed on the riser (where the riser is connected to the pipeline and the production facility) owing to liquid slugs arriving at the facility at high velocity with a tendency for the slugs to accelerate out of the pipeline.
The classical example of severe slugging occurs when three conditions are fulfilled. These are:
(i) a downwardly sloping pipeline;
(ii) a low velocity of the multiphase flow leading to a stratified fluid stream, and
(iii) the underwater pipeline connects to a riser.
Thus, the slope of the pipeline and the velocity of the multiphase fluid flow create conditions under which the stream in the flow line assumes a stratified-type flow pattern, with the gas flowing above the liquid. This segregation of the gas into the upper part of the inclined pipeline is the determining factor for the establishment of the severe-slugging phenomenon.
Severe slugging is characterized by a cyclical four-stage process, as follows:
Stage I: The multiphase flow does not have sufficient energy to transport the liquid up the riser and the liquid therefore accumulates at the base of the riser. Over time, a slug of liquid is formed, blocking the passage of gas. Gas pressure upstream of the blocking liquid slug increases forcing the forming liquid slug into the riser so that a head of liquid ascends through the riser. The column of liquid creates a hydrostatic pressure which increases as the column lengthens, this hydrostatic pressure being substantially equal to the increasing gas pressure in the pipeline upstream of the liquid slug. The length of the liquid slug may reach from 1 to several riser lengths in extreme cases.
Stage II: The upper level of the liquid slug reaches the top of the riser and the liquid begins to flow into the separator. At this point, the hydrostatic pressure is at a maximum.
Stage III: The increasing gas pressure behind the liquid slug becomes sufficient to overcome the hydrostatic head of the liquid and a gas pocket or bubble is forced into the lower end of the riser and begins to move up the riser. At this stage, there is essentially a fixed gas pressure acting on a diminishing head. The excess of gas pressure over the hydrostatic pressure causes the slug to shoot up the riser at high speed. The gas that has built up behind the liquid slug then discharges to the separator as a sudden gas surge (gas blow-through).
Stage IV: Once the gas surge has discharged into the separator, the multiphase flow has insufficient energy to carry the liquid up the riser (as Stage I). The liquid falls down the wall of the riser and begins to accumulate at the base of the riser thereby initiating a new cycle.
The slug of liquid followed by the gas surge can threaten to overwhelm the production facility. The gas blow-through is generally detected as a high pressure in the separator and a pressure detector means operates to cause valve means to close thereby shutting off the riser from the production facility and also closing down operation of the facility. This means that production is stopped whilst the effects of the severe slugging are dealt with and production may not be resumed for at least several hours thereby causing financial loss which can be exacerbated by the possibility of resumed production having to be initially at a low level and then progressively increased to the normal rate.
A number of approaches have been adopted to reduce or eliminate the effects of severe slugging. The use of a choke to control severe slugging is referred to in Oil and Gas Journal 12 Nov. 1979 at pages 230 to 238. Thus, stabilization of the flow may be achieved by introducing a localised pressure drop (head loss) due to a choke to the flow, at the top of the riser. The counter-pressure imposed by the choke at the surface is proportional to the velocity of flow past it. In this way, slugging is halted and the flow may be stabilized.
Gas lift has been employed to reduce the hydrostatic pressure of the column of liquid in the riser with a view to achieving a reduction in pressure in the line and keeping the liquid moving in the riser. This is achieved by injecting gas at the base of the riser. However, since the phenomenon of severe slugging cannot be readily controlled, it is generally necessary to inject large amounts of gas, which requires considerable compression means. Furthermore, injection of large amounts of gas substantially modifies the gas to oil ratio, which complicates the phase separation operations at the surface production facility.
GB 2 280 460 relates to an apparatus for inserting into a conduit such as a riser. The apparatus includes a tubular member having an external width less than the internal width of the conduit and mounting means to mount the tubular member within the conduit. The apparatus has the advantage that by reducing the flow volume of the conduit, slugging problems can be reduced and can usually be mitigated, especially when used in conjunction with gas lift.
U.S. Pat. No. 6,041,803 relates to a device for eliminating severe slugging in a stream in a flow line including a riser, the device comprising a body having a convergent nozzle section positioned in the flow line where the stream is about to enter the riser. The body further includes a divergent diffuser section with the convergent nozzle section and the divergent diffuser section defining an internal passage of the body with a geometric configuration such that it introduces a pressure drop which promotes a reorganization of phases in the multiphase flow, thereby converting the stratified flow pattern into a non-stratified flow pattern so that the multiphase stream passes up the riser.
U.S. Pat. No. 6,716,268 relates to a method of controlling occurrence of severe slugging in a riser that conveys a multiphase fluid system from a pipeline to a separation vessel for separation of a gas phase of the fluid system from a liquid phase. The separation vessel means is provided with a gas outlet comprising valve means to regulate speed of gas flow along the pipeline towards the riser in at least a vicinity of a lower end of the riser. The method comprises observing pressure in the pipeline at a position adjacent to said riser, and varying an extent to which the valve means is open so as to vary gas velocity in the pipeline to a value opposing or preventing occurrence of severe slugging in the riser.
However, there remains the need for further devices and methods for controlling the occurrence of severe slug flow.